1. Background of the Invention
This invention is directed to an apparatus, system, and method for extracting and enhancing an optical clock signal from an optical data signal. The optical data signal can be a non-return-to-zero (NRZ) or a return-to-zero (RZ) signal, for example. The apparatus and methods can be applied to extract and enhance the optical clock signal to recover the optical data signal, or for other purposes.
2. Description of the Related Art
FIG. 1A is a diagram for an optical clock signal. As can be seen in FIG. 1A, the optical clock signal is in general a sinusoidal signal with a fundamental period of xe2x80x9cT.xe2x80x9d Using the optical clock signal, a data signal can be encoded in different ways. Examples of encoding schemes are indicated in FIGS. 1B and 1C for return-to-zero (RZ) and non-return-to-zero (NRZ) signals, respectively. The RZ-formatted signal is such that it contains data pulses that return to zero in each bit period. In FIG. 1B, an exemplary byte of data xe2x80x9c01101001xe2x80x9d is encoded using the RZ signal format. The RZ signal has a xe2x80x9c1xe2x80x9d state upon the occurrence of a xe2x80x9c1xe2x80x9d in the data signal being encoded followed by a return to the xe2x80x9c0xe2x80x9d state in the later part of the period. The RZ signal remains xe2x80x9c0xe2x80x9d if the data bit is a xe2x80x9c0xe2x80x9d. In contrast, as shown in FIG. 1C, the non-return-to-zero (NRZ) signal will assume a xe2x80x9c1xe2x80x9d state upon the occurrence of a xe2x80x9c1xe2x80x9d in the data to be encoded in the entire duration of the bit period, and becomes xe2x80x9c0xe2x80x9d upon the occurrence of a xe2x80x9c0xe2x80x9d in the data to be encoded.
It will be appreciated that because the RZ and NRZ signals only change their state in synchronism with the optical clock signal, the RZ and NRZ optical data signals have some inherent information regarding the timing of the clock used in their generation. Determination of the clock signal used to generate the optical data signal is generally required to recover the optical data signal. There are many schemes for extracting a clock signal from the transmitted optical data signal at a receiver. For NRZ signals, a nonlinear operation is generally required for extracting a clock component since there is not clock component present in NRZ signals. When the nonlinear operation is performed in the electronic domain, an optical-to-electrical (OE) conversion is required. Such devices for OE conversion and the required nonlinear operation are relatively complicated in construction, and correspondingly costly to develop and manufacture. Another relevant consideration is that the conversion of a signal from optical to electronic form necessarily limits data processing rates due to inherent speed limitations of electronics. For future generations of optical networks, it is believed that such conversion will become a limiting factor in network performance. Hence, it would be desirable to provide an apparatus, system, and methods useful for extraction of an optical clock signal from an optical data signal without the need to convert to an electronic signals, to avoid attendant limitations in data rates imposed by such conversion. Furthermore, it would be desirable to enhance the clock component prior to performing all-optical clock recover for both the RZ and NRZ formats.
The disclosed invention in its various embodiments overcomes the above-noted disadvantages of previous devices and techniques, and achieves significant benefits over previous technologies.
A disclosed apparatus receives an optical data signal that can include data modulated on an optical carrier signal. The optical data signal can be a return-to-zero (RZ) signal or a non-return-to-zero (NRZ) signal, for example. The apparatus comprises a non-linear optical element (NLOE) and an optical frequency discriminator (OFD). The NLOE is coupled to receive the optical data signal, and generates a chirped signal based on the optical data signal. The OFD is coupled to receive the chirped signal from the NLOE. The OFD generates an optical clock signal based on chirped frequency components of the chirped signal. The NLOE can comprise a semiconductor optical amplifier, an optical fiber, or a non-linear crystal, for example. The OFD can comprise an optical filter for passing chirped frequency components of the chirped signal, but rejecting its non-chirp frequencies. The optical filter can comprise a grating such as a fiber Bragg grating (FBG) or a planar waveguide grating, for example. Alternatively, the optical filter can be implemented as a Fabry-Perot filter (FPF). The OFD can enhance the optical amplitude of the chirped frequency components of the chirped signal to produce the optical clock signal. More specifically, the optical data signal includes at least one optical pulse whose leading and trailing portions are chirped by the NLOE. The OFD can discriminate at least one of the blue-shifted and red-shifted components of the leading and trailing edges of the optical pulse(s), respectively, to generate the optical clock signal. The OFD can also suppress the optical carrier signal relative to the chirped frequency components of the chirped signal in generating the optical clock signal.
The NLOE can operate in transmission or reflection mode. If used in reflection mode, the apparatus can comprise a circulator, for transmitting the optical data signal to the NLOE and receiving the reflected chirped signal, and for supplying such chirped signal to the OFD. Similarly, the OFD can operate in transmission or reflection mode. In the reflection mode, the apparatus can comprise a circulator coupled to receive the chirped signal from the NLOE. The circulator can be coupled to supply the optical clock signal generated by the OFD using the chirped signal, to a downstream element.
The NLOE can be undirectional or bidirectional, meaning that light travels though the NLOE in one or both directions, respectively. Unidirectional mode can be used for chirping an optical data signal with the NLOE. In either unidirectional or bidirectional operation, the NLOE can be used to mediate the optical data signal""s cross-modulation of the CW signal to produce the chirped CW signal supplied as the chirped signal to the OFD. This provides the capability to extract a clock signal independently of the wavelength of the optical data signal. In the unidirectional case, the optical data signal and chirped CW signal travel in the same direction through the NLOE, and in the bidirectional case, such signals travel in opposite directions.
In one configuration, the apparatus can comprise a continuous wave (CW) source generating a CW signal. The CW source can be coupled to supply the CW signal to the NLOE. The NLOE can be an SOA or other element that mediates the optical data signal""s cross-modulation of the CW signal to produce chirped frequency components in the CW signal. The resulting chirped CW signal is supplied as the chirped signal to the OFD. The OFD can discriminate chirped frequency components of the chirped CW signal to enhance the frequency components indicating the timing of the optical clock signal used to modulate data onto the optical carrier signal to produce the optical data signal.
A disclosed system receives an optical data signal. The system comprises a clock extraction apparatus having a non-linear optical element (NLOE) and an optical frequency discriminator (OFD). The NLOE is coupled to receive the optical data signal, and generates a chirped signal based on the optical data signal. The OFD is coupled to receive the chirped signal from the NLOE. The OFD generates an optical clock signal based on chirped frequency components of the chirped signal. The system also comprises a clock recovery circuit coupled to receive the optical clock signal, or a signal based thereon. The clock recovery unit generates a recovered clock signal based on the received signal. The system comprises a decision circuit coupled to receive the recovered clock signal and a signal based on the optical data signal. The decision circuit generates an electric data signal representing data extracted from the optical data signal. The recovery circuit can be electronically- or optically-based. The system can comprise one or more optical-to-electronic (O/E) converters to transform the optical data signal and the optical clock signal into electronic form for supply to the decision circuit.
A disclosed method comprises generating a chirped signal based on an optical data signal, and discriminating chirped frequency components in at least one of the leading and trailing portions of one or more pulses in the chirped signal from non-chirp frequency components of the chirped signal, to produce an optical clock signal. The optical data signal can be a return-to-zero (RZ) signal or non-return-to-zero (NRZ) signal, for example. The chirping can be performed with a non-linear optical element (NLOE). The NLOE can be a semiconductor optical amplifier (SOA), an optical fiber, or a non-linear crystal, for example. The NLOE can operate in either transmission or reflection mode, and can be unidirectional or bidirectional. The discriminating can be performed with an optical frequency discriminator (OFD). The OFD can comprise an optical filter that can be implemented as a fiber Bragg grating (FBG), a planar waveguide grating, or a Fabry-Perot filter (FPF), for example. The OFD can operate in either transmission or reflection mode. The method can comprise generating a continuous wave (CW) signal, and the chirping can be performed by using the optical data signal to cross-modulate the CW signal. The chirped CW signal can be used as the signal subjected to discrimination in the method. The chirping and discriminating can be performed so as to enhance optical amplitude of the chirped frequency components of the chirped signal to produce the optical clock signal. Furthermore, the discriminating can be performed to suppress non-chirp frequency components of the chirped signal relative to the chirped frequency components of the chirped signal in generating the optical clock signal. The non-chirped frequency components of the chirped signal can be those of the optical carrier signal or the CW signal, for example. The discriminating can be performed on at least one of the blue-shifted and red-shifted components of the leading and trailing edges of the optical pulse, respectively, to generate the optical clock signal.
These together with other features and advantages, which will become subsequently apparent, reside in the details of construction and operation of the invention as more fully hereinafter described and claimed. In the description, reference is made to the accompanying drawings, which form a part of this document, in which like numerals refer to like parts throughout the several views. The drawings are not necessarily to scale, emphasis instead being placed upon illustration of the principles of the invention.